1. Field of the Invention
This invention relates to a wound R-C network, and more particularly to a wound R-C network which has a stable frequency response over a wide frequency range, and which is compact in size, easy to fabricate and inexpensive to manufacture.
2. Description of the Prior Art
In various types of electrical and electronic circuits, such as telephone switching systems, resistor-capacitor networks have proven to be very effective in protecting relay contacts against degradation due to arcing as a result of electrical surges in the equipment. Typically, in a basic form of relay contact protection, an R-C network is connected across the contacts of a relay coil so that when the contacts open to de-energize the coil, the network shunts the high frequency energy induced by the collapsing electric field of the coil about the contacts, and the energy is absorbed by the capacitor of the network to prevent arcing across the contacts. When the contacts are reclosed to energize the coil, the resistor of the network limits the low frequency energy flow from the capacitor through the contacts, whereby the network again prevents arcing across the contacts.
One form of a wound R-C network which is suited for this basic type of relay contact protection is disclosed in the U.S. pat. No. 3,786,322, issued Jan. 15, 1974, to D. R. Brown et al, in which the network is fabricated so as to have a frequency dependent impedance, in that it exhibits a high impedance at low frequency to limit the energy flow from the capacitor when associated relay contacts close, and a low impedance at high frequency to facilitate absorption of the electrical energy by the capacitor when the contacts open. In this patent, as two metallized dielectric strips are wound to form a resistor-capacitor roll, a serpentine resistance path is formed in the metal layer of each strip by removing lines of metal therefrom. Subsequently, the ends of the network roll are spray soldered, the network roll is wax-impregnated, and electrical lead connections are made at the opposite ends of the roll with end portions of respective ones of the conductive paths, to form the R-C network.
In certain electronic switching system applications, however, the resistor-capacitor networks perform the additional function of generating a waveform which is compared with an optimum waveform to self-test the system continuously during its operation, to insure that the system is functioning properly. Further, because of the high speed at which the electronic system operates, contact protection networks which do not respond electrically in a uniform manner over a wide frequency range, such as 60 cycles to 100 kilohertz, could adversely affect contact make-and-break times, causing the system to malfunction. Accordingly, it is essential that R-C networks for these applications exhibit a uniform degree of capacitance at least at the lower frequencies (e.g., up to 10 kilohertz), where the networks are primarily reactive, and a uniform effective series resistance at the higher frequencies (e.g., above 10 kilohertz), where the networks are primarily resistive.
A highly stable R-C network suitable for this purpose in electronic switching systems is disclosed in the U.S. pat. No. 3,534,248 issued Oct. 13, 1970 to J. C. Houda, Jr. et al, in which the resistance of the network is provided by a prefabricated discrete resistor. The resistor is manually disposed within an elongated aluminum cylinder with a terminating lead of the resistor projecting axially from the cylinder, while the other lead of the resistor is dressed along the outer periphery of the cylinder. A capacitor then is formed about the cylinder by winding layers of dielectric and electrically conductive material about the cylinder, with a first layer of electrically conducting material in electrical contact with the dressed resistor lead. During the winding of the final turns of the capacitor, after the first layer of electrically conducting layers has been terminated, a second terminating lead is manually inserted and secured between adjacent turns of the dielectric material in electrical contact with a second electrically conductive layer of material. A primary object of the subject invention is to provide a wound R-C network which has electrical stability comparable to this discrete resistor-type network, and which is compact in size, easy to fabricate and inexpensive to manufacture.